Gripping head with hold back for equalized jaw advancement



Feb. 15, 1966 c, J H 3,234,773

GRIPPING HEAD WITH HOLD BACK FOR EQUALIZED JAW ADVANCEMENT Filed Nov. 19, 1962 5 Sheets-Sheet 1 Bfw- ATTORNEY.

C. J. BATH Feb. 15, 1966 GRIPPING HEAD WITH HOLD BACK FOR EQUALIZED JAW ADVANCEMENT 5 Sheets-Sheet 2 Filed NOV. 19, 1962 INVENTOR.

BY A

2? ATTORNEY.

Feb. 15, 1966 J BATH 3,234,773

GRIPPING HEAD WITH HOLD BACK FOR EQUALIZED JAW ADVANCEMENT Filed NOV- 19, 1962 5 Sheets-Sheet 3 BY 2 Q 4;

A ATTORNEY I.

c. J. BATH 3,234,773

GRIPPING HEAD WITH HOLD BACK FOR EQUALIZED J'AW ADVANCEMENT Feb. 15, 1966 5 Sheets-Sheet 4 Filed NOV. 19, 1962 INVENTOR. G 4, M, Y

l" I I 2 ATTORNEY Feb. 15, 1966 C. J. BATH 3,234,773

GRIPPING HEAD WITH HOLD BACK FOR EQUALIZED JAW ADVANCEMENT Filed Nov. 19, 1962 5 Sheets-Sheet 5 IN VENTOR.

. ATTORNEY.

United States Patent Ofilice 3,234,773 Patented Feb. 15, 1966 3,234,773 GRIPPING HEAD WITH HOLD BACK FOR EQUALIZED J AW ADVANCEMENT Cyril J. Bath, Chagrin Falls, Ohio, assignor to The Cyril Bath Company, Solon, Ohio, a corporation of Ohio Filed Nov. 19, 1962, Ser. No. 238,403 4 Claims. (Cl. 72-493) This invention relates to stock gripping heads for gripping opposite margins of a metal sheet for applying stretch forming tension thereto adequate to tension the sheet into a range above its elastic limit, and particularly to a stretch forming head for use in pre-draw stretching of sheet metal for rendering the sheet more readily deformable in companion drawing dies.

For purposes of illustration, the invention is shown in a form which can readily be incorporated in an apparatus such as disclosed in United States application of Cyril J. Bath, Serial No. 173,310, filed February 14, 1962, and now Patent No. 3,116,780, issued on January 7, 1964, entitled Prestretch Fixture and Combination Thereof With Drawing Die Press, its use for other purposes being readily apparent from the illustrative example.

In stretching sheet metal for the above described purpose, the sheet is gripped at two opposite margins by stretch heads, respectively, and tensioned unidimensionally. Each stretch head has a pair of gripping jaws each of which must be at least as long as the margin it is to grip. For forming articles of substantial size, each of these margins frequently is as long as 96 inches and upwardly. It is desirable that the clamping force applied by the jaws to the margin of the sheet be uniform throughout the length of the jaws. However, the gripping force required for wide sheets of substantial thickness is enormous, and consequently the closing force usually applied is at a mechanical advantage provided by cams or toggles arranged so that the jaws are open in a retracted position and are wedged into closing relation as they move from retracted to extended position. To obtain a high mechanical advantage, the cams or toggles are such that the jaws have a very large component of movement parallel to the plane of the sheet and a very small component normal to the plane.

Each of the jaws of a pair is preferably coextensive lengthwise of the jaws with the length of the margin to be gripped and moves as a unit into engagement with the margin exposed toward it. However, the force applied to the jaws must be applied thereto at a number of locations distributed along the length of the jaws concurrently. It is desirable that the forces applied at these separate locations equalize substantially so as to assist in causing the jaws to move at all times parallel to their theoretically exact clamping positions in which their length is exactly 90 to the tensioning dimension. However, heretofore such exactitude has not been obtainable consistently. The force applied at any one of these cations for moving the jaws toward extended gripping position is subject to slight variations with respect to that applied at the others, due to small manufacturing discrepancies in the force applying mechanisms. An individual adjustment of each cannot readily be made and, if made, cannot long be retained. Accordingly, the set of jaws advances faster at one end of its gripped margin than at the opposite end. Since, however, the jaws of a set approach the sheet at a relatively flat angle as they advance and grip it, the more advanced end of the set engages the sheet before the less advanced end, consequently, the more advanced end pushes the gripped end of the margin lengthwise of the tensioning dimension toward the opposite set of jaws and warps the sheet slightly between the margins at the advanced end while the sheet is still flat at the less advanced end. The warp or curvature flatwise is a maximum at the more advanced end of the jaws and gradually decreases until it ceases to exist before reaching the opposite end. When the sheet is thus gripped, then, upon stretching, the sheet at the less advanced end is stretched considerably before the slack of the warpage is taken out of the sheet at the more advanced end. Finally, by the time the more advanced end is placed under stretching tension above its elastic limit, the portion of the sheet nearer the less advanced end of the jaws has been stretched to a greater degree or perhaps overstretched to near its ultimate strength.

It is an object of the present invention to provide a jaw aligner for assuring that the jaws of a set start in a proper position in which they extend, lengthwise of the jaws, at right angles to the direction of tensioning and parallel to the plane of the sheet and that they advance parallel to their starting position to assure that one end of the set does not advance toward extended and gripping position faster than the other end, and that both ends of the set advance toward extended position and arrive at clamping engagement with the sheet simultaneously with each other over their entire lengths.

A more specific object is to apply hydraulic force to the jaws, at a plurality of locations along their length for moving them toward extended position and to yieldably restrain the jaws so as to assure that the hydraulic pressure is the same at all of the locations before permitting initial movement of the jaws from fully retracted toward gripping position.

Various other objects and advantages will become apparent from the following description wherein refer-, ence is made to the drawings, in which:

FIG. 1 is a diagrammatic top plan view of a stretch head embodying the principles of the present invention;

FIG. 2 is a rear elevation of the head illustrated in FIG 1;

FIG. 3 is a fragmentary enlarged front elevation of the head illustrated in FIG. 1;

FIG. 4 is an enlarged vertical cross sectional view taken on the line 4-4 in FIG. 3;

FIG. 5 is an enlarged side elevation of the head illustrated in FIGS. 1 to 3;

FIG. 6 is a fragmentary top plan View of the structure illustrated in FIG. 5, showing the connection of the restraining means to the jaws; and

FIG. 7 is a view similar to FIG. 5 showing a modification of the restraining means.

Referring to the drawings, each gripping head includes a set of jaws 1, comprising an upper jaw and a lower jaw. These jaws are elongated and adapted to grip an elongated margin of a sheet of stock S which, for purposes of illustration, is shown as a sheet 72 inches wide. The ja ws preferably extend unbrokenly the full length of the margin which they are to grip so each ja w moves as a unit. For moving the jaws, they may be mounted in a suitable body 2 which, for convenience in adapting the invention to different lengths of margins to be gripped, may be composed of a plurality of sections 2A arranged in a row and extending lengthwise of the jaws. Each jaw 1 comprises an elongated strip of hardened metal having a gripping surface exposed toward the other jaw and adequate by closure of the two jaws 1 under pressure to grip and hold the sheet for the purposes hereinbefore described.

For supporting the jaws and applying adequate pressure thereto, each jaw is mounted on a plurality of carriers 5, the carriers being arranged end .to end and being, as an assemblage, coextensive end'wise with the jaws 1 Each jaw is secured to all of its associated holders by suitable keyways 6 and bolts 7, and each jaw connects all of its carriers together so that they act as a rigid structure. The jaws are arranged for movement edgewise to extended clamping relation and to retracted released rotation.

Since the jaws are to be closed under extremely high pressure, mechanical advantage must be provided for forcing them toward each other during their movement toward extended position. For this purpose, force multiplying means are provided. The ja w carriers are mounted in their respective sections 2A by bolts 8 which support the carriers for sliding movement toward extended position to the left in FIG. 4 and to retracted position, respectively. In the form illustrated, the force multiplying means are cooperating cam surfaces 9 and 10 in the body and on the carriers, respectively. The cam surfaces converge in a direction in which the jaws are moved for clamping so that as the jaws are moved to extended position, they gradually approach each other facewise, but with a large forward component of movement, until they fully engage the margin of the stock in clamping relation.

Power means are provided for moving the carriers of each section forwardly toward the extended position of the jaws. For this purpose, the carriers are provided with slots 13 which received heads 15 on piston rods 16. Upon reverse movement of the rods 16, they move the carriers to retracted position. Since the carriers are long endwise of the jaws, it is preferable to provide a plurality of rods uniformly distributed along the length of the carriers.

Three rods are provided for each carrier, thus making a total of nine rods for each set of jaws. The rods 16 are connected, respectively, to pistons 20 reciprocable in cylinders 21. Hydraulic pressure can be supplied to the cylinders .21 through suitable ducts 22, respectively, at the rods ends of the pistons so as to retract the jaws and to the head ends of the cylinders by suitable ducts 23, respectively, for driving the jaws into clamping engagement with the sheet margin. Hydraulic fluid is supplied for operating the jaws from a suitable source, such as a pump 24, driven by a motor 25, and having its pressure side 26 connected to a suitable reversing valve 27. Suitable packing glands 28 are provided about the piston rod 16.

In the form illustrated, adapted for a 72 inch wide. sheet, the pistons 20 preferably are eight inches, center to center, and inset four inches from the ends of the body. These cylinders are about 3% inches inside diameter and supplied by a pump which supplies hydraulic pressure thereto about 1100 pounds per square inch. The cam surfaces have a slope of about 15. Accordingly, the clamping pressure developed is very great.

In order to cause the pressure fluid to be introduced to the cylinders approximately concurrently, it is introduced through suitable manifolding.

As illustrated, the m-anifolding may be provided by means of transverse passages 30 and 31, respectively, passage 30 of each section 2A being connected to all of its ducts 23, thus connecting its cylinders in parallel at their head ends. Perhaps 31 of a section are connected to all of its ducts 22, thus connecting the rod ends of its cylinders in parallel. The passage 30 of each section is connected near its ends to fittings 32, respectively, which, in turn, are connected in series to a pipe 33. Thus, when the pressure is introduced in the pipe 33, it flows to the manifold 30 and is distributed to all of the ducts 23 of the section. Correspondingly, the passage 31 is connected at each of its ends to fittings 34 which, in turn, are connected to a line 35 so that the rod ends of the cylinders are connected in parallel and to the return line of the section. All the lines 33 and 35 are common to all sections. The line 33 is connected to the valve 27 through a line 36, and the line 35 is connected to the valve 27 4 through the line 37, as illustrated diagrammatically in FIG. 4.

It is apparent from the description that as the jaws move toward the extended position in which they grip the sheet S, as illustrated in FIG. 4, they necessarily approach each other so that their gripping faces are moving both with a small component normal to the sheet and a large component parallel to the plane of the sheet. Hence, if one end of the jaws engages the margin of the sheet before the other end, it tends to push the engaged portion forward, thus cocking the sheet and causing it to warp before the jaws engage the other end. Upon stretching, this necessarily requires that the unwarped end be tensioned to a greater degree to remove the warp before the warped end is under tension. This lead of one end over the other continues so that the original warped end is stretched into a range above its elastic limit before the other end is stressed appreciably. In any event, the result is a tendency of a wrinkle in the sheet which is very difficult to remove. Accordingly, it is desirable that the opposite ends of the set of jaws move together simultaneously exactly the same distance from fully retracted position.

The retracted positions can be selected by a provision of shoulders 38 on the rear of the carriers and shoulders 39 on the sections, the shoulders 38 and 39 being so arranged that the leading edges of the retracted jaws are parallel and aligned in a direction forwardly and rearwardly of the path of movement by the pistons and are precisely at right angles to the tensioning dimension.

The pistons and cylinders are accurately made and frictional resistance of the packing gland 28 may be adjusted, and frictional resistance to movement of the carriers on the cam surfaces may be made precisely. Assuming there is very small, but precisely equalized, friction in the pistons themselves, no piston would move in the jaw extending gripping direction before pressure is built up in all of the cylinders sufficient to move the pistons. But such precision is impractical. Even if they could be made so that frictional resistance were entirely eliminated, differentials in friction would be built up in the lines supplying pressure fluid to the manifold and in the manifold. Consequently, when pressure fluid is introduced in the pipe lines and manifold, it may reach one cylinder before the other. Generally, due either to differentials in frictional resistance in the flow of the fluid or in the movement of the pistons and carriers, one piston at one end of the body 2, being freer than the one at the other end, will move forwardly before the one at the other end, thus cocking the set of gripper jaws, so that their leading edges extend other than at a right angle to the tensioning dimension. This is true even when the differential in pressure, due to frictional resistance to the flow of fluid and frictional resistance to movement of the pistons, is very slight compared to the total pressure that can be built up finally. Regardless of the possible degree of pressure that might be finally obtainable it cannot build up to a substantial degree until the resistance imposed by the pistons is equalized.

Repeatedly adjusting the frictional resistance of each piston or metering its fiuid supply is unsatisfactory, because slight ditferences in temperature, pressure, or in foreign matter lodged in a cylinder may unbalance the adjustment. The resistance to movement of each piston is very small until its associated jaws come in contact with the sheet and start gripping it. When the jaws come in contact with the sheet, the pressure quickly rises to an amount that so predominates the flow resistance and resistance of the pistons and carriers that they cease to be a factor. But, by this time, it is too late to equalize the advance of the jaws.

Accordingly, in order to cause the jaws to advance equally and engage properly with their leading edges parallel to their starting position so that the length of the jaws is at right angles to the direction. of tension, a

yieldable hold-back for yieldably resisting movement of the jaws to extended position is provided.

Referring to FIGS. 5 and 6 the hold-back is in the form of four tension springs arranged one spring at each end of each jaw. Since the springs at opposite ends of the jaws are the same, those at one end only will be described in detail.

Referring to FIGS. 5 and 6, each spring is arranged at the outboard end of one of the outboard sections 2A so as to be exposed at the end of the body 2 for access for adjustment. The spring is connected at one end to its associated section end through the medium of eye bolt 41 which is slidably receivable through a hole 42 in a. bracket 43. The bracket 43 is secured in fixed position on the body 2. The eye bolts are threaded and are provided with adjusting nuts 44 as a result of which they can be adjusted axially for adjusting the tension of the spring. For connecting the spring operatively to its associated end of the jaw, a suitable pin 45 is secured in fixed position to the associated jaw 1, preferably by being connected to the carrier 5. The pins and the hole 42 are preferably so arranged that the axis of the spring 40 extends parallel to the direction of tension or to the axis of the pistons. Each spring 40 has a value such that the resistance to advance of the carriers which it imposes so greatly predominates the frictional resistances of the carriers and pistons that these resistances become negligible and without operative effect. Thus the springs can resist move ment of the carriers by the pressure fluid supplied to the cylinders until that pressure fluid is built up to a degree far above that which could be frictionally resisted by any piston, cylinder and carrier combination.

Subsequently, when pressure fluid is admitted, no jaw advances until the pressure fluid has become equalized to all cylinders and increased substantially so that it can overcome the springs. By the time this occurs, the frictional resistance to movement of the carriers, pistons and cylinders, and of the flow of the fluid have become such a negligible factor that all pistons advance simultaneously and force the carriers forwardly so that the jaws 1 advance at all times parallel to their retracted position.

For this purpose, in the form illustrated, each spring has a preloaded tension of about pounds and is capable of building up in value to about 35 pounds when the jaws are near their fully advanced position. However, the leading edges of the jaws are properly aligned and parallel at least by the time they move forwardly one half the distance necessary for engaging the sheet. Generally, it is satisfactory if all springs are of approximately equal value because by the time enough pressure has built up to overcome even the weakest of them it has equalized and then almost instantly rises to such a degree that the yieldable resistance imposed by the springs 40 is negligible. However, in case of excessive binding at some portion or the other, the springs may be adjusted for temporarily offsetting this binding effect.

Thus, when fluid is first introduced to the head end of the pistons, the springs exert efficiently yieldable resistance to the advance of the jaws so that no piston moves forwardly until the pressure fluid equalizes on all of the pistons. Since the volume of flow is very small for closing the jaws, and the pressure of the fluid is equalized at 1100 pounds per square inch almost instantly, by the time it is built up sufficiently to overcome the total spring resistance of 80 pounds, the spring resistance becomes negligible and has no operative effect on the movement of the pistons. The entire operation is almost instantaneous.

Referring next to FIG. 7, dashpots, such as indicated at 50, may be used and substituted one dashpot for each spring. Each dashpot preferably comprises a cylinder VII 51 in which is operable a piston 52 having a rod 53. The rod is provided with an eye 54 and the cylinder is provided with an eye 55, the eye 54 being operable to receive the pin 45, as heretofore described, and the eye 55 adapted for connection to a suitable pin on the body of the stretch head. A reservoir 56 containing pressure fluid, preferably hydraulic fluid, is connected by a port 57 to the rod end of the cylinder 51 by means of a pipe line 58. The reservoir is connected to the head end of the cylinder 51 by means of a pipe 60. An adjustable throttling valve 61 is interposed in the line 58 between the reservoir 56 and the port 5-7 in order to p redetermine the yieldable resistance to movement of the jaws by their piston and cylinder assemblages. No return spring or means is necessary in the dashpot inasmuch as the piston and cylinder assemblages for closing the jaws also act as the means for retracting the jaws.

Additional means may be used for yieldably opposing the advance of the jaws, the important feature being that the yieldable opposition is adequate to prevent any forward movement of any carrier until the pressure built up in all of the cylinders is equalized and is in excess of that necessary to overcome the resistance imposed by the yicldable hold-back means.

Having thus described my invention, I claim:

1. A stock gripping stretch head comprising an elongated rigid body, a pair of elongated complementary gripping jaws supported thereby for concurrent movement, edgewise of their clamping faces and laterally of the body, toward extended and retracted positions, respectively, force multiplying means for moving the jaws so that their clamping faces approach each other consequent upon, and during, movement of the jaws concurrently toward extended position, a plurality of re versible piston and cylinder assemblages connected to the jaws at locations distributed along the lengths of the jaws, circuit means connecting the cylinders in parallel with each other and adapted for conveying pressure fluid to the cylinders for applying force for moving the jaws concurrently to extended positions and concurrently to retracted positions, selectively, yieldable resistance hold back devices connected to the jaws of the pair, respectively, near the ends of each jaw, respectively, said yieldable resistance hold back devices having suflicient resistance to constrain the initiation of movement of the jaws toward extended position by the force when said force is initially applied by the assemblages until said force builds up in all cylinders to a predetermined higher force sufficient to cause movement of the piston having the highest frictional resistance to movement in the extending direction of the jaws, and having insufficient resistance to constrain initiation of movement of the jaws toward extending position by said higher force.

2. A structure according to claim 1 wherein said resistance of the hold back devices is greater than the frictional resistance of the heads, assemblages and force multiplying means, to movement of the heads toward extended position.

3. A structure according to claim 1 wherein the yieldable resistance hold back means are dashpots.

4. A structure according to claim 1 wherein at least one device of each jaw is adjustable for changing its resistance.

References Cited by the Examiner UNITED STATES PATENTS 2,218,503 10/1940 Brooks et a1 l53-33 2,352,442 6/1944 'Loewy et al. 15333 2,522,319 9/1950 Tinley 15335 3,082,809 3/1963 Petsch et a1 153-35 CHARLES W. LANHAM, Primary Examiner. 

1. A STOCK GRIPPING STETCH HEAD COMPRISING AN ELONGATED RIGID BODY, A PAIR OF ELONGATED COMPLEMENTARY GRIPPING JAWS SUPPORTED THEREBY FOR CONCURRENT MOVEMENT, EDGEWISE OF THEIR CLAMPING FACES AND LATERALLY OF THE BODY, TOWARD EXTENDED AND RETRACTED POSITIONS, RESPECTIVELY, FORCE MULTIPLYING MEANS FOR MOVING THE JAWS SO THAT THEIR CLAMPING FACES APPROACH EACH OTHER CONSEQUENT UPON, AND DURING, MOVEMENT OF THE JAWS CONCURRENTLY TOWARD EXTENDED POSITION, A PLURALITY OF REVERSIBLE PISTON AND CYLINDER ASSEMBLAGES CONNECTED TO THE JAWS AT LOCATIONS DISTRIBUTED ALONG THE LENGTHS OF THE JAWS, CIRCUIT MEANS CONNECTING THE CYLINDERS IN PARALLEL WITH EACH OTHER AND ADAPTED FOR CONVEYING PRESSURE FLUID TO THE CYLINDERS FOR APPLYING FORCE FOR MOVING THE JAWS CONCURRENTLY TO EXTENDED POSITIONS AND CONCURRENTLY TO RETRACTED POSITIONS, SELECTIVELY, YIELDABLE RESISTANCE HOLD BACK DEVICES CONNECTED TO THE JAWS OF THE PAIR, RESPECTIVELY, NEAR THE ENDS OF EACH JAW, RESPECTIVELY, SAID YIELDABLE RESISTANCE HOLD BACK DEVICES HAVING SUFFICIENT RESISTANCE TO CONSTRAIN THE INITIATION OF MOVEMENT OF THE JAWS TOWARD EXTENDED POSITION BY THE FORCE WHEN SAID FORCE IS INITIALLY APPLIED BY THE ASSEMBLAGE UNTIL SAID FORCE BUILDS UP IN ALL CYLINDERS TO A PREDETERMINED HIGHER FORCE SUFFICIENT TO CAUSE MOVEMENT OF THE PISTON HAVING THE HIGHEST FRICTIONAL RESISTANCE TO MOVEMENT IN THE EXTENDING DIRECTION OF THE JAWS, AND HAVING INSUFFICIENT RESISTANCE TO CONSTRAIN INITIATION OF MOVEMENT OF THE JAWS TOWARD EXTENDING POSITION BY SAID HIGHER FORCE. 